The spinal trigeminal nucleus is further subdivided into three parts, from rostral
to caudal:

* Pars Oralis

* Pars Interpolaris

* Pars Caudalis

There is also a distinct trigeminal motor nucleus that is medial to the chief sensory
nucleus.

The mesencephalic nucleus is involved with proprioception of the face, that is,
the feeling of position of the muscles. Unlike many nuclei within the CNS, the mesencephalic
nucleus contains no chemical synapses but are electrically coupled.Instead, neurons
of this nucleus are pseudounipolar cells receiving proprioceptive information from
the jaw, and sending projections to the motor trigeminal nucleus to mediate monosynaptic
jaw jerk reflexes. It is also the only structure in the CNS to contain the cell bodies
of a primary afferent, which are usually contained within ganglia (like the trigeminal
ganglion).

The principal sensory nucleus (or chief sensory nucleus of V) is a group of second
order neurons which have cell bodies in the dorsal Pons.

It receives information about discriminative sensation and light touch of the face
as well as conscious proprioception of the jaw via first order neurons of CN V.

* Most of the sensory information crosses the midline and travels to the contralateral
ventral posteriomedial (VPM) of the thalamus via the Ventral trigeminothalamic tract.

* However, information of the oral cavity travels to the ipsilateral Ventral
Posteriomedial (VPM) of the thalamus via the Dorsal trigeminothalamic tract.

The spinal trigeminal nucleus is a nucleus in the medulla that receives information
about deep/crude touch, pain, and temperature from the ipsilateral face. The facial,
glossopharyngeal, and vagus nerves also convey pain information from their areas
to the spinal trigeminal nucleus.

Motor branches of the trigeminal nerve

Motor branches of the trigeminal nerve are distributed in the mandibular nerve. These
fibers originate in the motor nucleus of the fifth nerve, which is located near the
main trigeminal nucleus in the pons. Motor nerves are functionally quite different
from sensory nerves, and their association in the peripheral branches of the mandibular
nerve is more a matter of convenience than of necessity.

In classical anatomy, the trigeminal nerve is said to have general somatic afferent
(sensory) components, as well as special visceral efferent (motor) components. The
motor branches of the trigeminal nerve control the movement of eight muscles, including
the four muscles of mastication.

* Muscles of mastication

* masseter

* temporalis

* medial pterygoid

* lateral pterygoid

* Other

* tensor veli palatini

* mylohyoid

* anterior belly of digastric

* tensor tympani

With the exception of tensor tympani, all of these muscles are involved in biting,
chewing and swallowing. All have bilateral cortical representation. A central lesion
(e.g., a stroke), no matter how large, is unlikely to produce any observable deficit.
However, injury to the peripheral nerve can cause paralysis of muscles on one side
of the jaw. The jaw deviates to the paralyzed side when it opens.the reason is lower
motor neuron paralysis of nerve causes loss of activity, where as upper motor neuron
as it has bilateral even though it has damage in one side of cortex the other side
helps in function as lower motor neuron is intact.

Somatotopic representation

Onion Skin Distribution of the Trigeminal Nerve

Exactly how pain/temperature fibers from the face are distributed to the spinal trigeminal
nucleus has been a subject of considerable controversy. The present understanding
is that all pain/temperature information from all areas of the human body is represented
(in the spinal cord and brainstem) in an ascending, caudal-to-rostral fashion. Information
from the lower extremities is represented in the lumbar cord. Information from the
upper extremities is represented in the thoracic cord. Information from the neck
and the back of the head is represented in the cervical cord. Information from the
face and mouth is represented in the spinal trigeminal nucleus. [

Within the spinal trigeminal nucleus, information is represented in an onion skin
fashion. The lowest levels of the nucleus (in the upper cervical cord and lower medulla)
represent peripheral areas of the face (the scalp, ears and chin). Higher levels
(in the upper medulla) represent more central areas (nose, cheeks, lips). The highest
levels (in the pons) represent the mouth, teeth, and pharyngeal cavity.

The onion skin distribution is entirely different from the dermatome distribution
of the peripheral branches of the fifth nerve. Lesions that destroy lower areas of
the spinal trigeminal nucleus (but which spare higher areas) preserve pain/temperature
sensation in the nose (V1), upper lip (V2) and mouth (V3) while removing pain/temperature
sensation from the forehead (V1), cheeks (V2) and chin (V3). Analgesia in this distribution
is “nonphysiologic” in the traditional sense, because it crosses over several dermatomes.
Nevertheless, analgesia in exactly this distribution is found in humans after surgical
sectioning of the spinal tract of the trigeminal nucleus.

The spinal trigeminal nucleus sends pain/temperature information to the thalamus.
It also sends information to the mesencephalon and the reticular formation of the
brainstem. The latter pathways are analogous to the spinomesencephalic and spinoreticular
tracts of spinal cord, which send pain/temperature information from the rest of the
body to the same areas. The mesencephalon modulates painful input before it reaches
the level of consciousness. The reticular formation is responsible for the automatic
(unconscious) orientation of the body to painful stimuli.

Wallenberg syndrome

Wallenberg syndrome (also called the lateral medullary syndrome) is a classic clinical
demonstration of the anatomy of the fifth nerve. It provides a useful summary of
essential points about the processing of sensory information by the trigeminal nerve.

A stroke usually affects only one side of the body. If a stroke causes loss of sensation,
the deficit will be lateralized to the right side or the left side of the body. The
only exceptions to this rule are certain spinal cord lesions and the medullary syndromes,
of which Wallenberg syndrome is the most famous example. In Wallenberg syndrome,
a stroke causes loss of pain/temperature sensation from one side of the face and
the other side of the body.

The explanation involves the anatomy of the brainstem. In the medulla, the ascending
spinothalamic tract (which carries pain/temperature information from the opposite
side of the body) is adjacent to the descending spinal tract of the fifth nerve (which
carries pain/temperature information from the same side of the face). A stroke that
cuts off the blood supply to this area (e.g., a clot in the posterior inferior cerebellar
artery) destroys both tracts simultaneously. The result is loss of pain/temperature
sensation (but not touch/position sensation) in a unique “checkerboard” pattern (ipsilateral
face, contralateral body) that is entirely diagnostic.